The following account of the prior art relates to one of the areas of application of the present application, hearing aids.
The primary purpose of a hearing aid (HA) is to amplify sound regions which would otherwise be inaudible to a hearing impaired user. In quiet, non-reverberant surroundings a state-of-the-art HA succeeds, and the user is usually able to hear and understand everything.
However, in reverberant environments, e.g. rooms with hard surfaces, churches, etc., the ability to understand speech declines. This is so because the signal from the target speaker is reflected on the surfaces of the environment; consequently, not only the direct (un-reflected) sound from the target speaker reaches the ears of the HA user, but also delayed and dampened versions are received due to the reflections. The larger and “harder” a room is, the more reflections. In the extreme case, the reflections are perceived as an echo.
The amount of reverberation in a room can be broadly characterized through a single quantity called T60 and defining a characteristic reverberation time. T60 is defined as the time required for reflections of a direct sound to decay by 60 dB below the level of the direct sound. The larger the value of T60, the more reverberation in the room. T60 and its measurement are defined in ISO 3382-1:2009.
Recently, a class of signal processing algorithms have been introduced which, in principle, are capable of processing the reverberant signal picked up at the microphone and produce a processed signal where the reverberation has been reduced, see e.g. [Habets, 2007] or [Löllmann, 2009] and the references therein. Ideally these algorithms would use the T60 to adjust the processing, but since this quantity is not known in general, it is estimated from the reverberant signal. Unfortunately this is difficult to do in a reliable manner, and the algorithms are rather sensitive to a decent estimate of T60. Had the T60 been known to the signal processing algorithms somehow, performance would be significantly better.
US 2009/208043 A1 describes an external device capable of wirelessly transmitting information about the local acoustic environment to a hearing aid. US 2004/213415 A1 describes a method of determining reverberation, e.g. for use in a hearing aid.